1. Field
The techniques and devices described herein relate generally to integration of optical data communication technology and electronic circuits, and relate particularly to an optical receiver which is suitable for use with integrated silicon photonic interconnect technology and is compatible with a standard CMOS process.
2. Discussion of the Related Art
Many electronic devices use one or more integrated circuits to receive, store, process, and/or send data. An integrated circuit (“IC” or “chip”) may include a wafer of semiconductor material, such as silicon, on which one or more electronic circuits have been fabricated by applying a sequence of processing steps to the semiconductor wafer. These processing steps may include, for example, photolithographic patterning, material deposition, doping, annealing, material removal, and cleaning. For reasons that are understood by one of ordinary skill in the art (e.g., power dissipation, scalability, and/or cost of manufacturing), many ICs are fabricated using a standard CMOS (complementary metal-oxide semiconductor) manufacturing process, rather than a customized CMOS process or a non-CMOS process.
An electronic or optoelectronic device may use data communication technology to move data from one location to another within the device, or to exchange data with another device. A variety of data communication technologies are known, including electrical and optical technologies. Electrical data communication technologies may transport data by propagating electrical signals through metal interconnects (e.g., wires). Optical data communication technologies may transport data by propagating optical signals (e.g., light) through optical interconnects (e.g., waveguides).
Optical data communication technologies that propagate optical signals through a silicon medium are known as silicon photonic systems. In a silicon photonic system, the plasma dispersion effect may be used to control the concentration of free charge carriers in a semiconductor device, thereby modulating the light carried by a nearby optical waveguide. The concentration of free-carriers may be controlled by carrier injection, carrier depletion, or carrier accumulation techniques.
Although silicon's refractive index is only weakly dependent on the concentration of free charge carriers, a ring resonator structure for enhancing this dependence is known See, for example, Lipson, Nature 2004, p. 1082. Use of such a ring resonator structure may facilitate low-power optical modulation in silicon.
An optical data communication system may include an optical modulator and an optical receiver. An optical modulator may encode data in an optical signal by modulating one or more of the signal's properties, such as its phase, amplitude, frequency, or polarization. Such modulation may be achieved by changing an optical property of the waveguide through which the optical signal propagates, such as the waveguide's absorption coefficient or refractive index.
An optical receiver may convert an optical signal into an electrical signal. Optical-to-electrical signal conversion may be performed using a photodetector, such as a photodiode. A photodiode may produce an electrical signal (e.g., a current) in response to an optical signal that passes through or near the photodiode. Some photodiodes may produce not only a photocurrent (e.g., a current indicative of the intensity of the detected optical signal) but also a dark current (e.g., a current indicative of thermal generation or leakage processes intrinsic to the photodiode and independent of any optical signal illuminating the photodiode).
An optical receiver's performance may be characterized using various performance metrics, such as data rate (e.g., the amount of data that can be received in a given time period, sometimes expressed in Gb/s), energy dissipation (e.g., the amount of energy dissipated by the receiver per amount of data received, sometimes expressed in fJ/bit), and sensitivity (e.g., the minimum input current that can be reliably detected by the portion of the optical receiver circuit that is downstream from the photodetector(s), sometimes expressed in μA).